/Engine/Plugins/Animation/RigLogic/Source/RigLogicLib/Public/dna/layers/GeometryWriter.h
The vertex position indices must be stored in the same order as the deltas they are associated with.
void setBlendShapeTargetVertexIndices ( std::uint16_t meshIndex, std::uint16_t blendShapeTargetIndex, const std::uint32_t * vertexIndices, std::uint32_t count )